I imagine a common scenario is a couple arriving home after a significant amount of drinking and feeling amorous (fueled by the alcohol). Perhaps the guy doesn't think he has to pee just then, but the beer is still cranking through his kidneys, and by the time he's well into the process his bladder is very full. If he stops to take a leak, the mood may be gone, so he keeps at it, maybe even hoping he cums soon so he can go take a leak!